Piston-driven flow of highly concentrated suspensions

The piston-driven flow of highly concentrated suspensions (55 to 62% by volume solids) was investigated experimentally and theoretically. In the experiments, the effect of piston speed (0.13 to 26 mm/s), liquid viscosity (1 to 55,000 cP), particle material (silica glass or PMMA), particle size (0.5 or 1.4 mm), and added solid phase compressive stresses (0 to 2400 Pa) on the flow resistance were examined. A frictional and a viscous regime of flow resistance were identified along with a relatively sharp transition between them. For the frictional regime, models based on differential slice analysis for quasi-static granular beds and Coulomb friction were used to successfully correlate results for suspensions with interstitial fluid viscosities in the range of 1 to 641 cP and zero compressive stress. However, in some cases, the single parameter Kfi that characterized the drag appeared to be larger than expected for the active Rankine stress condition. The transition to viscous behavior was observed for suspensions of 1.4 mm glass particles in 3200 cP liquids at a piston speeds near 6.5 mm/s. This transition coincided with conditions under which elastohydrodynamic lubrication theory predicted that the height of the lubrication layer between the particles near the channel wall just exceeded the height of surface roughness features of the sliding surfaces. Below the piston speed or viscosity of the transition, frictional drag behavior was observed. At higher viscosities or piston speeds, behavior characteristic of viscous drag was observed, including increasing drag with increasing piston speed, and the drag was near that predicted with hydrodynamic suspension models. The piston forces were a direct measure of the drag, but liquid pressure drop measurements did not correlate with the drag, and appeared to indicate the fraction of particles freely suspended in the interstitial fluid. Two-dimensional DEM simulations tested a broad range of friction factors and demonstrated high friction cases in which a significant portion of the particles did not contribute to the drag of the suspension and could presumably be in a freely suspended state. In addition, the DEM results indicated the presence of passive Rankine stress zones at high friction factors, which would explain the high Kfi values observed experimentally under some conditions.